In a typical communications network, also referred to as e.g. a wireless communications network, a wireless communications system, a communications network or a communications system, a wireless device communicates via a Radio Access Network (RAN) to one or more Core Networks (CNs).
The wireless device may be a device by which a subscriber may access services offered by an operator's network and services outside the operator's network to which the operator's radio access network and core network provide access, e.g. access to the Internet. The wireless device may be any device, mobile or stationary, enabled to communicate over a radio channel in the communications network, for instance but not limited to e.g. user equipment, mobile phone, smart phone, sensors, meters, vehicles, household appliances, medical appliances, media players, cameras, Machine to Machine (M2M) device or any type of consumer electronic, for instance but not limited to television, radio, lighting arrangements, tablet computer, laptop or Personal Computer (PC). The wireless device may be portable, pocket storable, hand held, computer comprised or vehicle mounted devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another wireless device or a server.
The wireless device is enabled to communicate wirelessly in the communications network. The communication may be performed e.g. between two wireless devices, between a wireless devices and a regular telephone and/or between the wireless device and a server via the radio access network and possibly one or more core networks and possibly the Internet.
The radio access network covers a geographical area which is divided into cell areas. Each cell area is served by a base station, e.g. a Radio Base Station (RBS). In some radio access networks, the base station is also called evolved NodeB (eNB), NodeB or B node. A cell is a geographical area where radio coverage is provided by the base station at a base station site. The base station communicates over the air interface operating on radio frequencies with the wireless device(s) within range of the base station.
LTE Background
LTE is short for Long Term Evolution and is a technology which uses Orthogonal Frequency Division Multiplexing (OFDM) in the DownLink (DL) and Discrete Fourier Transform (DFT)-spread OFDM in the UpLink (UL). Uplink is communication going up from the wireless device to the base station and downlink is communication going down from the base station to the wireless device. OFDM is a method of encoding digital data on multiple carrier frequencies and used in LTE to schedule resources in both the frequency and time domain. DFT-spread OFDM, also referred to as DFTS-OFDM, is a transmission scheme that may combine the desired properties for uplink transmission i.e.:                Small variations in the instantaneous power of the transmitted signal.        Possibility for low-complexity high-quality equalization in the frequency domain.        Possibility for Frequency Division Multiple Access (FDMA) with flexible bandwidth assignment.        
Due to these properties, DFT-spread OFDM has been selected as the uplink transmission scheme for LTE.
The basic LTE downlink physical resource may thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element 101 corresponds to one subcarrier during one OFDM symbol interval on a particular antenna port. The resource element 101 is the smallest unit within OFDM, which is one OFDM symbol including cyclic prefix transferred on one carrier. Cyclic prefix is used to prefix a symbol with a repetition of the end. The receiver may discard the cyclic prefix. The cyclic prefix serves the purpose as a guard interval to eliminate interference from the previous symbol and as a repetition of the end of the symbol. An antenna port is defined such that the channel over which a symbol on the antenna port is conveyed may be inferred from the channel over which another symbol on the same antenna port is conveyed. There is one resource grid per antenna port. The carrier spacing is 15 kHz, and is used for broadcast and multicast.
LTE downlink transmissions are organized into radio frames of 10 ms in the time domain. Each radio frame comprises ten equally-sized subframes of 1 ms as illustrated in FIG. 2. A subframe is divided into two slots, each of 0.5 ms time duration.
The resource allocation in LTE is described in terms of resource blocks, where a resource block corresponds to one slot in the time domain and twelve contiguous 15 kHz subcarriers in the frequency domain. Two in time consecutive resource blocks represent a resource block pair and corresponds to the highest granularity time interval upon which scheduling operates.
Scheduling is a mechanism where a wireless device requests a network node for the resource allocation during each Transmission Time Interval (TTI). If the wireless device has some data that it needs to transmit continuously, it will request the network node e.g. every TTI, for the resource allocation. This scheduling type may be referred to as dynamic scheduling. The advantage of dynamic scheduling is flexibility and diversity of resource allocation. Using other words, scheduling refers to selection of which wireless device(s) is/are to use the radio resources at each TTI, where one TTI is e.g. 2 ms.
To allow the wireless device to request uplink transmission resources from the network node, LTE provides a Scheduling Request (SR) mechanism. The scheduling request conveys a single bit of information, indicating that the wireless device has data to transmit to the network node.
The scheduling mechanism may be implemented by a scheduler in the network node which assigns the time and frequency resources among wireless devices. A Resource Block (RB) is the smallest element that may be assigned by the scheduler. A downlink physical resource is represented as a time frequency resource grid comprising multiple resource blocks. A resource block is divided in multiple Resource Elements (RE). The scheduler may base its assignment decision on Quality of Service (QoS) information provided by e.g. the wireless device, queuing delay of the data to be transmitted, channel conditions etc.
Because LTE is based on OFDM, it is possible to distribute available transmission resources in the frequency domain to different wireless devices. This allocation may be changed dynamically once per subframe, that is, once per millisecond. The Medium Access Control (MAC) scheduler in the network node is in charge of assigning and scheduling both uplink and downlink radio resources for different wireless devices and their services. The scheduling decision covers not only the resource block assignment but also which modulation and coding scheme to use and whether or not to apply Multiple Input Multiple Output (MIMO) or beam forming.
Transmissions in LTE are dynamically scheduled in each subframe where the base station transmits downlink assignments and/or uplink grants to certain wireless devices, e.g. user equipments, via the physical downlink control information, i.e. Physical Downlink Control CHannel (PDCCH) and evolved PDCCH (ePDCCH). The PDCCHs are transmitted in the first OFDM symbol(s) in each subframe and spans more or less the whole system bandwidth. A wireless device that has decoded a downlink assignment, carried by a PDCCH, knows which resource elements in the subframe that comprises data aimed for the wireless device. A downlink assignment is an assignment of an allocated radio resource to the wireless device. Similarly, upon receiving an uplink grant, the wireless device knows which time/frequency resources it should transmit upon. In LTE downlink, data is carried by the Physical Downlink Shared data CHannel (PDSCH) and in the uplink the corresponding link is referred to as the Physical Uplink Shared CHannel (PUSCH).
The work with defining the enhanced downlink control signaling (ePDCCH) is ongoing in the Third Generation Partnership Project (3GPP). However, it is likely that such control signaling may have similar functionalities as PDCCH, with the fundamental difference of requiring wireless device specific DeModulation Reference Signal (DMRS) instead of Cell-specific Reference Signals (CRS) for its demodulation. One advantage is that wireless device specific spatial processing may be exploited for ePDCCH. DMRS is a physical signal used for coherent demodulation of uplink data and control signaling. CRS is used for both demodulation and measurement purposes.
Multi-TTI Scheduling
As to reduce the scheduling assignment/grant overhead one feature being discussed for inclusion in Release 12 of LTE is multi-TTI scheduling. The TTI in LTE is 1 ms, which corresponds to one subframe. A multi-TTI scheduling assignment/grant indicates to a wireless device that the wireless device is to receive or transmit data involving multiple TTIs. This should not be confused with Semi Persistent Scheduling (SPS), which primarily is used to effectively support low rate streaming services, such as voice calls. Semi persistent scheduling is a semi static allocation, configured by means of Radio Resource Control (RRC) messages. In case of semi persistent scheduling, the network node may assign a predefined chunk of radio resources for Voice over Internet Protocol (VoIP) wireless devices with an interval of 20 ms. Therefore, the wireless device is not required to request resources each TTI, saving control plan overhead. This scheduling is semi persistent in the sense that network node may change the resource allocation type or location if required for link adaptation or other factors.
On the contrary, multi-TTI scheduling is envisioned as a dynamic assignment that is dynamically indicated in a Downlink Control Information (DCI) format, including information of the resource block assignment in frequency. Hence, multi-TTI scheduling is operating at a much higher time granularity than SPS, and provides substantially increased flexibility to change the resource allocation in frequency. A DCI comprises uplink or downlink resource allocation. The PDCCH carries the resource assignment for wireless devices which are comprised in a DCI message.
Scheduling Restrictions
An essential aspect of base station implementation is to minimize the constraints imposed on the allowable scheduling. In particular, the data traffic is dynamic by nature and may change on a very short time scale. In particular, scheduling assignments/grants involving multiple subframes constrains the base station scheduling behavior in the upcoming subframes. For example, if new data reaches the base station, it may not be able to transmit this data until previous scheduling commitments are fulfilled. Such constraints introduce additional delays in the link which may be very detrimental for delay sensitive traffic. Scheduling constraints for uplink transmissions similarly degrades the performance and latency of the communications network. Scheduling restrictions inherently degrades the communications network's adaptability for changes in the radio environment and traffic load.
The existing solutions for multi-TTI scheduling involves mapping of a number of consecutive subframes, which limits the usability to scenarios where such transmissions are indeed suitable. This is not the case in systems in which the interference is coordinated among transmission points or in heterogeneous deployments employing enhanced Inter-Cell Interference Coordination (eICIC), i.e. cell range expansion, where transmission/reception is constrained to certain subframes.
Scheduling assignments/grants involving multiple TTIs, i.e. subframes, has the advantage of reduced scheduling assignment/grant signaling overhead, but comes at the cost of reduced dynamic scheduling flexibility.